The cities were selected based on size (more than 100,000 inhabitants to ensure statistical power) and their ability to represent the diversity of climates found in metropolitan France: oceanic (Bordeaux, Le Havre, Nantes, Rennes, and Rouen), modified oceanic (Lens-Douai, Lille, Paris, and Toulouse), semi-continental (Clermont-Ferrand, Dijon, Grenoble, Lyon, Nancy, and Strasbourg), and Mediterranean (Marseille, Montpellier, and Nice). They correspond to the largest French metropolitan areas and cover the entire metropolitan territory. These 18 cities have a combined population of over 15 million. We were thus able to study the influence of average daily temperatures ranging from -12 to +32°C.

As for the study period, it was chosen based on the availability of air pollution data (starting in 2000) and mortality data (through 2010) at the time the study began in 2014. The chosen period includes years prior to the implementation of the heatwave plan in 2004 and years after its implementation.

The study shows how exposure to a given temperature affects the risk of mortality for several days following that exposure. The statistical models used then make it possible to determine, for each city, the “optimal” temperature associated with the lowest mortality risk, and to calculate the number of deaths attributable to higher temperatures (“heat”) and lower temperatures (“cold”).

The “optimal” temperature ranges from 13°C in Lille to 22°C in Paris, highlighting the population’s adaptation to the local climate. The results thus show that the effects of temperature are not limited to extreme temperatures.

Between 2000 and 2010, 3.9% of total mortality in cities was attributable to cold and 1.2% to heat. The study shows that the impact of cold begins two days after exposure and persists for at least 21 days. Furthermore, very low temperatures account for only a small portion of cold-related mortality, which is more closely linked to moderate but frequent low temperatures. Conversely, the effect of heat on mortality peaks on the very day of exposure and lasts only a few days. Finally, while very high temperatures remain rare, they account for approximately 30% of the total burden of heat-related mortality. Above average temperatures ranging from 23 to 28°C depending on the city, each additional degree results in a very rapid increase in the risk of death, justifying specific action in the event of extreme heat.

Finally, no difference was found in the pattern and intensity of the temperature-mortality relationship before and after the implementation of the heat wave plan in 2004.

The goal is to extend the study period and include overseas territories (Martinique, Guadeloupe, French Guiana, Réunion), about which very little is known. It will also be interesting to include recent years, which have seen record-breaking heat. Globally, 2014, 2015, 2016, and 2017 were the four hottest years since meteorological observations began.

Santé publique France participates in the international Multi-City Multi-Country (MCC) network. Coordinated by the London School of Hygiene and Tropical Medicine, this network conducts similar analyses across more than 500 cities worldwide, making it the largest environmental health research network in the world. In particular, this network produces impact estimates based on various climate scenarios developed by the IPCC (Intergovernmental Panel on Climate Change). For French cities, the group’s latest publications show that by 2100, we can expect an increase of approximately 4% in temperature-related mortality under a warming scenario of about +3°C. Only a warming of less than +2°C—in line with the Paris Agreement’s target—would limit this rise in mortality.

From a prevention perspective, we are also working on complementary approaches aimed at describing the characteristics of people who die during heat waves, exploring the influence of urban heat islands, and examining the synergies between temperature and air pollution. This work is supported by collaborations with Météo-France, several organizations working on urban climatology, and Inserm.